Fixing of stiffeners onto panels in aircraft is generally achieved by riveting.
In order to reduce the time and cost of operations for fixing stiffeners onto corresponding panels, other techniques are envisaged including in particular the so-called friction stir welding technique.
The technique referred to as friction stir welding (FSW) is known, in general, for making fast durable mechanical joints which allow forces to pass between assembled parts with an efficiency which is at least equivalent to that achieved using a conventional riveted joint.
This technique, shown schematically in FIG. 1, uses a welding device which includes at least one welding head 10, which includes a rotating pin 12 and a shoulder 14 which extends to the base of the rotating pin 12 and which has a diameter which is typically equal to 2 to 2.5 times the mean diameter of this rotating pin 12.
Friction stir welding of two parts 16a, 16b involves introducing the rotating pin 12 into the two parts at the joint interface 18 between them until the shoulder 14 makes contact with the surface of each of the parts 16a, 16b. This introduction of the rotating pin 12 into the material making up the parts 16a, 16b, is made possible by local softening of this material as a result of the heating produced by the friction of the rotating pin 12 against the two parts 16a and 16b. The dough-like state of the material of the parts 16a, 16b around the rotating pin 12 then allows this rotating pin to move along the joint interface 18. The rotation of the rotating pin 12 as well as, if appropriate, that of the shoulder 14, causes stirring of the material in the dough-like state.
The extrusion caused by the rotating pin 12 and the forging effect produced by the shoulder 14 thus gradually results in the formation of a weld bead. This weld bead takes the form of a new metallurgical structure common to the two materials, formed as a result of recovery-recrystallization phenomena, which thus guarantees good cohesion of the two parts 16a, 16b after cooling.
As shown schematically in FIG. 2, a counter-pressure is applied on the face of each part 16a, 16b opposite the welding head 10, using a counter-form 20, in order to counteract the pressure exerted by the rotating pin 12. Such a counter-form sometimes incorporates a cooling device which allows part of the heat generated by the friction to be removed. This, in general, improves the mechanical properties of the parts after welding. Such a cooling device takes the form, for example, of channels 21 incorporated in the counter-form 20 and in which a heat-transfer fluid flows.
The friction stir welding technique allows so-called “butt” welding to be carried out, as shown in FIGS. 1 and 2, wherein the axis of the rotating pin 12 is locally parallel to the joint interface between the parts to be assembled.
This technique also allows so-called “transparency” welding to be carried out, in which the axis of the rotating pin 12 is locally orthogonal to the joint interface between the parts to be assembled. In this case one of the parts to be assembled is interposed between the welding head and the other part to be assembled.
The friction stir welding technique in particular exhibits the advantage of being carried out below the melting point of the constituent material of the parts to be assembled, which in particular avoids problems associated with re-solidification which usually occur with other welding techniques.
This technique in addition offers the advantage of not requiring any filler materials, and of not causing any emission of polluting fumes.
Furthermore the speed at which the welding device moves along the joint interface of the parts to be assembled may reach 2 meters per minute, so that this welding technique allows parts to be assembled quickly and at reduced cost.
This welding technique in addition offers possibilities for high levels of automation.
The application of this welding technique to the assembly of stiffeners and of aircraft panels poses a problem however.
In effect, in the conventional assembly of a stiffener and of an aircraft panel, a layer of mastic is usually placed between the stiffener and the panel, in order, in particular, to provide a seal between these elements and thus limit the risk of corrosion of these elements, and also in order to minimise wear phenomena resulting from the vibrations between the stiffener and the aircraft panel.
In the case of the friction stir welding technique, although the problem of vibrations may be less marked than that encountered with the riveted method of assembly, on the other hand the problem relating to the seal between the assembled parts persists.
This problem of the seal is illustrated in FIGS. 3, 3a and 3b. 
FIG. 3 is a partial diagrammatic view in longitudinal section of a circumferential frame 22 fixed to an aircraft fuselage panel 24 by a transparency friction stir welding process of a known type, with no use of mastic placed between the assembled parts.
FIG. 3a shows on a larger scale the joint between the flange 26 of the circumferential frame 22 and the fuselage panel 24 in the case of welding using two weld beads 28 relatively close to the web 30 of the circumferential frame 22.
FIG. 3b is a similar view to FIG. 3a, illustrating the case of a weld using two weld beads 28 which are relatively far away from the web 30 of the circumferential frame 22 and respectively close to the opposite edges 32a and 32b of the flange 26 of this circumferential frame 22.
In both cases the method of assembly does not prevent the appearance of micro-cavities 34 whose dimensions are exaggerated in these figures for the purposes of clarity. Such micro-cavities, widely referred to as “corrosion traps”, allow moisture to penetrate the joint between the parts and thus promote the corrosion of these parts.
In the case of friction stir welding, interposing a layer of mastic between a stiffener and an aircraft panels is not desirable, since the mastic mixes with the material constituting the stiffener and the panels during welding, resulting in a reduction in the structural qualities of the welded assembly.
This problem naturally occurs when the width of the flange to be welded is greater than the width of the footprint of the rotating pin in this flange, that is, greater than the diameter of the rotating pin and in particular when the width of the flange is greater than double the diameter of the rotating pin.